The goal of the proposed Project II is advancement of scientific understanding of language skills underiying reading and writing in older elementary and middle school students , development of a causal theory of change upon which to base instructional strategies and for students within general education classrooms, with potential implications also for students who struggle.This project (Project II) involves three related specific aims: Specific Aim 1 is to identify, through investigative experiments, mechanisms (each operating at a different level of language) by which adolescents' morphological skills play a role during comprehension and composition. We first explore three potential mechanisms via which morphological skill may contribute to comprehension: by facilitating 1) word recognition, 2) word learning/inferring the meanings of new words and thereby supporting vocabulary, and 3) analyzing the meaning and syntactic function of words within sentences to support comprehension. We also examine how corresponding aspects of morphological skill during text production contribute to writing skill. Specific Aim 2 is to examine the extent to which adolescents' use of these various morphological skills is related to reading and writing skill (including specific learning disabilities in written language identified in Projecti). In addition to students with the range of skill found in general education classes, students with SLDs referred from Project I will also participate in the investigative tasks in Project II, and differences in performance on Project II tasks will be examined in light of response to intervention rates established in Project I, providing additional information as to the role of morphological skill in reading skill, as well as RTI. Specific Aim 3 is to develop instructional interventions designed to increase students' morphological skills and examine effects on comprehension and composition. Such interventions will address issues of causality, which are beyond the scope of the investigative studies, by examining whether improvements in specific morphological skills (identified as part of Specific Aims 1 and 2) lead to improvements in reading and writing outcomes.